This invention relates generally to hardware interfaces between communication stations and various types of transmission media used in communication networks, particularly networks that employ an access protocol commonly referred to as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). One widely used CSMA/CD protocol is commonly known as Ethernet. The CSMA/CD protocol is defined in ANSI/IEEE Std 802.3, published by the Institute of Electrical and Electronics Engineers, Inc., 345 East 45th Street, New York, N.Y. 10017, and referred to in this specification as "the IEEE standards."
Modern communications employ a variety of communication media, including twisted pairs of conductors, optical fibers, and coaxial cables. The equipment at each network station must obviously be designed differently to interface with these different media, and the IEEE standards make a logical separation of station equipment into media-dependent and media-independent functional modules. The media-independent modules include a physical signaling module (PLS), a media access control module (MAC), and a logical link control module (LLC). The media-dependent station equipment module is referred to as the media attachment unit (MAU).
Since the IEEE standard protocol supports different communication media, vendors of station equipment must provide support for connection to many, and ideally all, media types. Each medium type has different specifications for its connector, signal levels, transmission line signal conditioning, electrical isolation, and safety requirements. One approach to station design would be to provide a universal station that included media attachment units (MAUs) for every conceivable communication medium, but this would require costly and inefficient use of input/output (I/O) panel space and circuit board space in the station equipment. Moreover, the necessary profusion of connector types on each station would tend to be confusing to the average equipment user.
A typical compromise in station equipment design is to provide two external connectors: one to connect directly to a widely used twisted-pair communication medium, and the other to connect to another selected media type, through an external media attachment unit (MAU). This is in conformance with the IEEE standards, which defines station equipment (or data terminal equipment, DTE) in two permissible ways. First, the DTE can be defined to include all media-independent and media-dependent equipment, right up to the medium itself. In this case, the DTE includes an integral media attachment unit (MAU) for connection to a commonly used medium, such as a twisted-pair medium. In the IEEE standards, this mode of operation with a twisted-pair medium is referred to as 10BASE-T operation (pronounced ten-base-tee), and the connector for coupling to the twisted-pair medium is referred to as a 10BASE-T connector. The other way that the DTE can be defined is to include only the media-independent equipment, and exclude the MAU. In this configuration, an MAU is usually, but not necessarily, an external piece of hardware. In accordance with the IEEE standards, the MAU is coupled to the media-independent station equipment through an interface referred to as the attachment unit interface (AUI). The AUI is defined in the standard to include a connector that can be coupled to a variety of off-the-shelf MAU modules that can be purchased to connect to various communication media.
Therefore, a useful configuration for station equipment includes a 10BASE-T connector for coupling directly to twisted-pair media, and an AUI connector for connecting to another selected medium, through an appropriate external MAU. Nevertheless, this configuration has some significant disadvantages. One is that the standard AUI interface uses what is referred to as a 15-pin Sub-D connector with a slide-latch, at the station end of the interface. This connector is relatively large, so uses considerable space on the station, and is not as reliable as other connector types. Perhaps more important is the difficulty that a customer can still encounter in configuring a station to connect to a particular network. Basically, the customer must still choose which connector to use.
One known solution to this problem is to combine the two types of connectors, AUI and 10BASE-T, into a single, larger connector assembly. This provides some degree of cabling simplification for the customer, but still requires knowledge of which network medium is being used, so that the station can be switched to the appropriate configuration.
Another useful two-connector configuration for station equipment includes an AUI connector and a 10BASE-F connector for connection a fiber-optic medium. Specifications for the 10BASE-F connector may be found in a Draft Supplement to ANSI/IEEE Std. 802.3-1990, dated Dec. 16, 1991, but the details of the connector design are not important to the present invention. Conceptually, the 10BASE-F proposed standard fiber optic media is similar to the 10BASE-T standard for twisted pair media. The station may also have a hardware-based or software-based switch for selecting the appropriate interface. In one arrangement, the user must connect the appropriate cable to one connector (AUI or 10BASE-F) and set the switch appropriately. In another arrangement, the user connects both cables, for redundancy, but must select which one to use by means of the switch. Since the user cannot be relied on at all times to select the appropriate cable and switch setting or, in the redundancy case, to select the switch setting only, this two-connector configuration presents the same difficulties as the one involving AUI and 10BASE-T connectors.
As will be appreciated from the foregoing, there is still a need for improvement and simplification in the manner in which station equipment is configured for connection to various network media. The present invention provides a simple yet elegant solution to this problem.